Gas turbine power plants are used as the primary power source for aircraft, in the form of jet engines and turboprop engines, as auxiliary power sources for driving air compressors, hydraulic pumps, etc. on aircraft, and as stationary power supplies such as backup electrical generators, for hospitals and the like. The temperatures and stresses to which most gas turbine engine components are subjected require that such components be fabricated of high strength, high temperature materials, such as superalloys and titanium alloys.
The design, construction and materials of rotors for gas turbine engines often dictate operating limits for the turbines in which they are employed. Extensive efforts have been made over the years to develop new alloys, new fabrication techniques, and new component designs which permit operation of these rotors at higher operating temperatures and/or lead to lighter weight, longer lived components, with all their attendant advantages. The most common rotor design used today in high temperature, high speed applications, such as in gas turbine engines for jet aircraft, comprises a disk with blades or airfoils mechanically attached to the rim thereof. The alloy used for the disk is selected to meet the requirements of high tensile strength and good low cycle fatigue resistance. Such properties are found in, for example, fine equiaxed grain superalloy materials. The airfoils, which are exposed to the higher temperatures of the gas path, as well as higher centrifugal loads, are stress rupture and creep limited. Thus, the blades are commonly produced from suitable materials having good stress rupture and creep resistance, such as coarse grained materials. No alloy processed to a singular microstructure can give optimum properties demanded by the conditions encountered in both the disk and airfoil sections without requiring excessive weight. One piece, integral centrifugal rotors, such as radial inflow turbine rotors, pose similar problems.
There are many techniques disclosed in the prior art for fabricating integrally bladed rotors using different materials for the blades and the hub or disk. The phrase "different materials", as used in this specification, refers to materials having different properties but which may or may not have the same elemental composition. Thus, alloys having the same composition but which are processed differently so as to exhibit differing properties are considered to be "different materials". One such technique was to diffusion bond blades of one material to a disk of another material, using hot isostatic pressing. In U.S. Pat. Nos. 4,096,615 and 4,270,256 it was recognized that maintaining precise dimensional controls between adjacent airfoil components was a difficult problem. Both patents include relatively complex procedures for forming an integral ring of blades, the radially inward facing surface of which is machined to a highly precise diameter to form a bonding surface with the radially outward facing surface of a rotor disk made from a different material. The ring and disk are positioned in mating relationship, oxygen and other contaminants are removed by vacuum outgassing, and external joint lines are sealed with braze material. Hot isostatic pressing is then used to diffusion bond the blades to the disk. Aside from the difficulties encountered in positioning the blades about the disk prior to hot isostatic pressing, prior art diffusion bonding techniques have not consistently resulted in satisfactory solid state bonds.
A more recent development is the Gatorizing.RTM. isothermal forging method useful with high temperature superalloys, as described in commonly owned U.S. Pat. No. 3,519,503, the teachings of which may be used in conjunction with commonly owned U.S. Pat. Nos.4,074,559 and 4,265,105, which describe apparatus which may be used to forge integrally bladed rotors from superalloys. Other patents relevant to the fabrication of dual material rotors include U.S. Pat. Nos. 2,479,039; 2,703,922; 2,894,318; 3,047,936; 3,598,169; 3,905,723; 4,051,585; 4,063,939; 4,097,276; 4,175,911; and 4,529,452.
A recent trend is to use single crystal turbine blades, as described for example in U.S. Pat. Nos. 3,494,709, 4,116,723, and 4,209,348, because of their exceptional high temperature mechanical properties. U.S. Pat. No. 4,592,120 suggests that such single crystal blades might be fabricated into an integrally bladed rotor by a casting method wherein a liquid metal would be poured into a mold containing preformed single crystal blades held in spaced alignment with a central disk portion. This approach has the inherent difficulty of limiting the disk rim properties to those of cast materials, whereas it is well known that forged materials provide better overall properties for disk applications.
Another technique for preparation of integrally bladed rotors is more fully described in a paper entitled "Fabrication and Heat Treatment of a Nickel-Base Superalloy Integrally Bladed Rotor for Small Gas Turbine Applications" by Hughes, Anderson, and Athey, published Jun. 22, 1980 in Modern Developments in Powder Metallurgy, Volume 14, Special Materials, published by Metal Powder Industries Foundation. In this paper the fabrication of an integrally bladed rotor by the previously mentioned Gatorizing process using a single alloy throughout is discussed.
Still another method for the preparation of integrally bladed rotors is disclosed in U.S. Pat. No. 4,529,452, wherein a turbine disk, made from a metal alloy which has been processed to exhibit superplastic properties at elevated temperatures, is diffusion bonded to turbine blades made from another metal alloy, by disposing the components in a press in mating contact. After forging at a high temperature, the surfaces are diffusion bonded. The new integral assembly is then heat treated to obtain the desired properties.
In U.S. Pat. 4,864,706, fabrication methods are described for producing an integrally bladed rotor wherein single crystal blade portions are securely metallurgically bonded to a polycrystalline disk. In this instance, individual single crystal blades are bonded to forged protrusions on the disk. In this and other prior art techniques, problems have been encountered in the fixturing and positioning of individual blades during solid-state diffusion bonding. Elaborate fixturing approaches can accommodate positioning requirements but suffer from low through-put since blades are generally individually fixtured and bonded, an extremely time consuming and manpower intensive procedure.